Conventionally, there is known a fuel cell system using a membrane electrode assembly (MEA) 90 as disclosed in Patent Document 1. This MEA 90 includes, as shown in FIG. 4, an electrolyte membrane 91 composed of a solid polymer membrane such as Nation (registered trademark of DuPont), a cathode 93 that is bonded to one surface of the electrolyte membrane 91 and supplied with air, and an anode 92 that is bonded to the other surface of the electrolyte membrane 91 and supplied with fuel such as hydrogen.
The cathode 93 is composed of a gas-permeable base material 93b, such as carbon cloth, carbon paper, or carbon felt, and a cathode catalyst layer 93a formed on one surface of the base material 93b. The other part of the cathode 93 than the cathode catalyst layer 93a is the base material 93b, which serves as a cathode diffusion layer that diffuses air to the cathode catalyst layer 93a on the nonelectrolyte side.
The anode 92 is also composed of a base material 92b similar to the base material described above, and an anode catalyst layer 92a formed on one surface of the base material 92b. The other part of the anode 92 than the anode catalyst layer 92a is the base material 92b, which serves as an anode diffusion layer that diffuses fuel to the anode catalyst layer 92a on the nonelectrolyte side.
The cathode catalyst layer 93a and the anode catalyst layer 92a include, as shown in FIG. 5, innumerable pieces of catalysts 81 each composed of a support 81a made of carbon black supporting catalyst metal fine particles 81b of a material such as platinum (Pt), and a polyelectrolyte 82 that binds the pieces of the catalysts 81 to each other and binds them to a base material (not shown). As the polyelectrolyte 82, a similar material to that of the electrolyte membrane 91 is used.
Then, the MEA 90 is sandwiched between separators (not shown) to form a fuel cell serving as a minimum unit of electric power generation, and then, a large number of the cells are stacked together to form a fuel cell stack. The cathode catalyst layer 93a is supplied with air by air supply means, and the anode catalyst layer 92a is supplied with hydrogen or the like by hydrogen supply means or the like. Thus, the fuel cell system is formed.
In the MEA 90, hydrogen ions (H+: protons) and electrons are generated from the fuel by an electrochemical reaction in the anode catalyst layer 92a. Then, the protons move in the electrolyte membrane 91 toward the cathode catalyst layer 93a in the form of H3O+ accompanied by water molecules. The electrons flow into the cathode catalyst layer 93a though a load connected to the fuel cell system. On the other hand, in the cathode catalyst layer 93a, water is generated from oxygen contained in air, the protons, and the electrons. The electrochemical reaction as described above occurs continuously, whereby the fuel cell system can generate an electromotive force continuously.